Phase pattern correction for transmitter having a radome

ABSTRACT

Apparatus for altering an electromagnetic wave phase configuration to a predetermined nonplanar front to compensate for radome phase distortion and which wave, upon exiting the radome, has a phase front which is planar.

3 13-8726 RR 3569975 5R [72] Inventor George C. Fretz, Jr. [56]References Cited Cuyahoga Falls, Ohio UNITED STATES PATENTS [21] f 22,609,505 9/1952 Pippard 343/754 [22] ed d 1971 2,975,419 3/1961 Brown343/755x ggi xa Aerospace Corporation 3,189,907 6/1965 Van Buskirk343/753 Akron O OTHER REFERENCES Jasik, Antenna Engineering Handbook,McGraw-Hill, N.Y., 196] TK7872A6J3 Pg. 32-2 and 32-21 relied on [54]PHASE PATTERN CORRECTION FOR P E P IL G ns] r TRANSMITTER HAVINGARADOMEZ' fg'i e e 2Claims,4Drawing Figs. 0mey ere [52] U.S.Cl 343/781,343/872, 343/840 [51] Int. Cl ..H0lq' 19/14, ABSTRACT: Apparatus foraltering an electromagnetic wave H0lq3/00,H01q 1/42 phase configurationto a predetermined nonplanar front to [50] Field of Search 343/753-compensate for radome phase distortion and which wave,

upon exiting the radome, has a phase front which is planar.

PHAE kA'lTERN CORRECTION FOR TRANSMITTER HAVING A RADGME A monolithicfoam construction for large spherical ground radomes has severaldesirable qualities over rival types. The radome does not require astructural frame of steel, aluminum, or even dielectric ribs thatprovide degrading blockage, reflection, and diffraction effects. The RFloss through the low density foam is small or nominal at the frequenciesof use. Beam bending or boresight error through a radome of this kindcan be maintained very low.

This kind of radome is, however, now without some of its own particularproblems. When a large radome of reasonable foam thickness is placedover an antenna, edge rays of the antenna will impinge on the radomesurface at a much higher angle of incidence than central rays. Thiscauses a differential phase delay across the area of the aperture forrays emanating through the radome. This phase taper can, for highfrequency radar antenna, and through thick foam sections become quitelarge and result in serious loss of gain'and considerable distortion ofthe radiated antenna pattern.

The general object of the invention is to overcome this phase problem inradomes by the provision of a simple, yet highly efficient solution tothe problem.

A further object of the invention is to provide for a phase correctionin a radiating antenna associated with the radome by mechanicallycontrolling the wave front of the emanating electromagnetic waves sothey approach the radome out of phase and thus leave the radome inphase.

The aforesaid objects of the invention and other objects which willbecome apparent as the description proceeds are achieved in a radomephase pattern correction system which comprises a parabolic antennareflector, a feed horn mounted to the center of the reflector andextending substantially perpendicularly from the surface thereof, and aradome surrounding and protecting the antenna and horn which ischaracterized by mechanically having the feed horn positioned slightlyaway from the focal point of the reflector or controlling the curvatureof the reflector, or utilizing some type of lens, whereby the wave formpropagated from the reflector is substantially parabolically shapeditself to an extend such that passage of the wave form through theradome emerges having a wave front of the desired characteristics.

For a better understanding of the invention reference should be had tothe accompanying drawings wherein:

FIG. 1 is a schematic illustration of a reflector and radomeincorporating the principles of the invention;

FIG. 2 is a schematic illustration showing the effect on the emanatingelectromagnetic wave forms dependent upon the position of the radiatingend of the horn with respect to the focal point of the reflector; and

FIG. 3 illustrates schematically the mechanical reconstruction of thereflection to achieve the objects of the invention; and

FIG. 4 illustrates a modified embodiment of the invention which utilizesa lens to obtain the wave form correction prior to impingement on theradome.

The solution to the problem of phase error in a radome set forth by thisinvention relates to a parabolic reflector with a point-source feed. Inessence, the system involves three separate mechanical embodiments toobtain wave form correction prior to impingement onto the radome. Thisprovides a phase advance similar in magnitude and opposite in sign tothe phase delay caused by the radome for the rays radiating through theradome. This correction can, in many cases, improve the antennaradiation pattern to almost the same characteristics as the originalfree space pattern.

With reference to the form of the invention illustrated in FIG. l of thedrawings, the numeral indicates generally a parabolic reflector having afeed horn l2 centrally positioned therein and extending substantiallyperpendicularly from such center position surface. The normal focalpoint of reflector 10 is at point 14, marked by the X, but to achievethe purposes of the invention, the actual radiating point of the horn isat the end l6. In essence, the radiation of electromagnetic energydirected back towards reflector 10 from the radiating end 16 of the feedhorn 12 will produce an electromagnetic wave form emanating in awavefront looking like dotted line 18. The wave form 18 impinges againsta radome 20, which normally is made from a thick, low density foam, allin a manner well understood by those skilled in the art. The waveform18, upon passing through the radome 20, is phase corrected nonlinearlyalong its length depending upon increasing divergence from a normalentrance into the radome 20 so that in effect the wave front 18 flattensout to form a straight or planar front indicated by dotted front 18a. Inthe normal radiation, which would occur from focal point 14,. the wavefront transmitted would be planar, and would be bent back to the reverseparaboloid shape 18!) because of the phase delay present in the radome20.

The phase delay increases in magnitude for rays further away from thecenter of the reflector 10. By moving the feed point 16 away from thefocal point 14, the rays traveling from the feed to points on the dishfurther from its centerline are provided an increasing phase advance.This effect is opposite to but not necessarily equal to the phase delaycaused by the radome 20.

The calculation of the distance of displacement making a total feeddistance from the reflector 10 equal to distance p, as contrasted to thefocal distance f, is critical to the proper operation of the invention.Calculation of this distance reveals that the following relationshipshould be present:

(p-f) (1-cos 1) where A CD (mm phase advance for the ith ray due to thefeed displacement f= distance from the vertex to the focal point p=distance from the vertex to the new feed point; and

l angle from the antenna centerline to the ith ray from the feed.

Thus, for example where a 4 -foot diameter parabolic reflector with aconical feed horn having a focal point 14 inches from the vertex of theparabola, an increase in distance of 0.085 inches would be optimum withan antenna feed horn aperture of 0.72 inches in diameter with about-12.5 and l4.5 decibels of energy at the edge of the reflector in the H-plane and E-planes, respectively.

FIG. 2 illustrates the theory of this embodiment of the invention inmore readily understood detail. Specifically, a

parabolic reflector 30 having a feed horn 32 is mounted and surroundedby a randome 34. The radiating end of the horn is indicated at positions1, 2, and 3, with position 2 being the optimum normal focal point of thereflector 30. Radiation of the feed horn 32 with the radiating elementat point 1 will result in a phase front indicated by dotted line 1 infront of the radome 34. Similarly, the wave fronts for positions 2 and 3are illustrated by dotted lines. However, note the phase fronts indicated by dotted curves 1a, 2a, and 3a differ considerably because ofthe phase distortion present in the radome 34 upon the transmission ofthe waves therethrough. It should thus be understood that the optimumcondition is for wave form 3a to be the resultant emanating wave form,and hence movement of the radiating point away from the reflector 30according to the formula above to achieve proper defocusing provides thesolution to the corrections required.

This correction or compensation technique is considered hereparticularly for a thick foam radome at a high microwave frequency. Thecompensation is, however, applicable to other frequencies, kinds, andsizes of radomes (including fiberglass laminate and honeycomb sandwich)and antenna where phase distortion of the RF signal transmitted throughthe radome, due to its contour and thickness, becomes a problem.

FIG. 3 illustrated a modified embodiment of the invention where thecontour of a reflector 50 is conformed to provide a wave front indicatedby dotted line 54, which upon passage through radome 52 take the flatplanar shape indicated by dotted line 54a. Here the horn 56 ispositioned at an optimum focus condition but the contour of thereflector 50 gives the appropriate predetermined correction necessary towavefront FlG. 4 illustrates another mechanical embodiment of theinvention which is a lens 60 positioned between a reflector 62 and aradome 64 so as to provide the desired wavefront configuration indicatedby dotted line 66. Preferably, the lens 60 will be made of a suitablematerial such as the same material as the radome 64 so as to delay thosewaves at the center for longer times than those waves nearer to theedges. This proportional delay will provide the wavefront 66, asindicated.

While in accordance with the patent statutes only the best knownembodiment of the invention has been illustrated and described indetail, it is to be particularly understood that the invention is notlimited thereto or thereby, but that the inventive scope is defined inthe appended claims.

l claim:

l. Electromagnetic wave phase-changing apparatus comprising:

a. a radome;

b. first means transmitting electromagnetic wave energy;

and

0. second means operatively mounted relative to the radome and saidfirst means to alter the wave phase to a nonplanar configuration tocompensate for radome phase distortion and produce an exiting wavehaving a predetermined phase front, and characterized in that the secondmeans comprises an antenna reflector, and the first means comprises anelectromagnetic wave feed means operatively positioned out of the focalpoint of the reflector to alter the wave phase configuration to anonplanar front, whereby the waveform exiting from the radome has apredetermined phase front.

2. A system according to claim 1 where the distance the feed horn shouldbe from the reflector is determined by the formula ACE (mm phase advancefor the ith ray due to the feed horn displacement f= distance fromvertex to the focal point for the reflector p= distance from the vertexto the new feed point W angle from the antenna centerline to the ith rayfrom the feed.

1. Electromagnetic wave phase-changing apparatus comprising: a. aradome; b. first means transmitting electromagnetic wave energy; and c.second means operatively mounted relative to the radome and said firstmeans to alter the wave phase to a nonplanar configuration to compensatefor radome phase distortion and produce an exiting wave having apredetermined phase front, and characterized in that the second meanscomprises an antenna reflector, and the first means comprises anelectromagnetic wave feed means operatively positioned out of the focalpoint of the reflector to alter the wave phase configuration to anonplanar front, whereby the waveform exiting from the radome has apredetermined phase front.
 2. A system according to claim 1 where thedistance the feed horn should be from the reflector is determined by theformula Delta (F.D.) (p-f) 1-cos Psi where Delta (F.D.) phase advancefor the ith ray due to the feed horn displacement f distance from vertexto the focal point for the reflector p distance from the vertex to thenew feed point Psi angle from the antenna centerline to the ith ray fromthe feed.